Congestion-based network traffic policing based on congestion exposure is a promising network resource control paradigm that accounts for user traffic in the event of network congestion. It has been argued, for example in B. Briscoe, “Flow Rate Fairness: Dismantling a Religion”, ACM Computer Communications Review, 37(2), 63-74 (April 2007), that flow rate fairness, which has been used in the past, is not a reasonable mechanism for resource allocation and accountability of network resources. Instead, it is suggested that a cost-based mechanism would provide a better resource allocation paradigm, in which “cost” means the degree to which each user's transfers restrict other transfers, given the available network resources. The metric that has been suggested for measuring this cost is the volume of network congestion caused by each user. A network traffic policing mechanism based on congestion offers a net-neutral way for network operators to manage traffic on their networks.
There have been several proposals for implementing congestion-based network traffic policing. For example, Re-ECN (Relay or Re-feedback of Explicit Congestion Notification) is a proposal that has been made in the Internet Engineering Task Force (IETF) Congestion Exposure (CONEX) Working Group, being described, for example, in B. Briscoe, A. Jacquet, C. Di Cairano-Gilfedder, A. Salvatori, A. Soppera, and M. Koyabe, “Policing Congestion Response in an Internetwork using Re-feedback”, Proc. ACM SIGCOMM'05, CCR, 35(4):277-288, August 2005. As will be discussed in greater detail below, Re-ECN, or re-feedback of explicit congestion notification provides a feedback mechanism through which packets expose the congestion that they expect to cause. The key feature is the user based accountability that is not based on resource usage but on the congestion user traffic is causing to others in a network. It is important to note that Re-ECN is one way of doing Congestion Exposure mechanism and alternatives are also possible.
For instance, in a Re-ECN system as shown in FIG. 1, which is a specific implementation of a congestion exposure system, there are different functional entities: routers detect congestion and apply Explicit Congestion Notification (ECN) to packets in their queue. Receiving endpoints collect this congestion information and relay it back to the sender. The sender runs a transport protocol (for instance TCP) and can use this information for the congestion control algorithm of a transport protocol. Also, the sender is expected to re-act to the received feedback declaring its contribution to congestion for subsequent packet transmissions. This is done by marking a certain fraction of packets appropriately. An operator-provided Congestion Exposure aware Policer can use this information to police or to account for traffic accordingly.
Thus, the Congestion Exposure approach is based on the idea that senders adapt to congestion indications from the network, for instance by decreasing TCP sending rates, and that senders declare their current congestion contribution to the network, which enables the network, e.g. routers, hosts, policers etc., to see the current congestion on a path. For instance, such information can be used to police traffic based on its congestion contribution according to some operator policy.
Since such policing can result in charging or other operator measures, users are normally not interested to over declare their current congestion contribution. In the Congestion Exposure framework, there are additional entities in form of an Audit Function that can enforce that congestion contribution is also not under declared in order to avoid non-conformity, e.g. cheating. In case non-conformity is detected, an Audit Function would normally start dropping packets of identified non-conformant flows to enforce a sending rate reduction for that particular flow. In summary, in Congestion Exposure, it is important for senders to declare their congestion contributions to the network correctly.
Furthermore, this approach does work in different scenarios, e.g. in fixed or mobile communication scenarios. For incorporating Congestion Exposure into mobile communication networks, a major challenge is to handle path changes caused by new network attachments. There are essentially two main issues:                Associating with a new wireless access point that changes the paths for current flows to/from a mobile node. The new paths may exhibit a lesser or greater congestion, but the current sending behavior and the current congestion declaration is based on earlier received feedback and may not match the new paths' conditions. In particular, if a mobile user moves to a higher congestion region, he may inadvertently create deficit at the egress Audit Function that in turn may classify that particular subset of traffic as non-conformant.        When multiple access networks are available for a handover decision, the mobile node has currently no means to select the most optimal one with respect to average path congestion. Some mobility architectures base handover decision on current access point load, but that is not sufficient if the congestion is occurring at different locations in the path.        
In view of the above, it is therefore an object of the present invention to improve and further develop a method of the initially described type for supporting congestion management in a congestion exposure-enabled network in such a way that, by employing mechanisms that are readily to implement, mobile users have an improved quality of experience in the context of handling path changes in events of mobility.